The sum to 2n terms of the series
\(\frac 34 + \frac 74 + \frac{15}{16} + \frac{31}{16} + \frac{63}{64} + \frac{127}{64} + ..........\) is
(a) \(3n - \frac 23(1 - \frac1{4^n})\)
(b) \(3n - \frac {10}{21}(1 - \frac1{4^n})\)
(c) \(3n - \frac {13}{21} \frac1{4^n}\)
(d) None of these
